1. Field of the Invention
The present invention relates to a driving circuit of a motor, which is used to drive a motor, particularly, an ultrasonic motor.
2. Description of the Related Art
There has conventionally been known an ultrasonic motor which employs ultrasonic vibration as driving force. In a traveling-wave ultrasonic motor which is a kind of ultrasonic motor, a stator is formed with a piezoelectric body attached to a ring-shaped elastic body and a rotor mounted on a driving shaft press-contacts the stator. The driving circuit of the ultrasonic motor supplies, to the piezoelectric body, driving signals of two phases (sine wave and cosine wave) which differ from each other by 90 degrees at a predetermined frequency. Due to mechanical vibration of the piezoelectric body caused by the driving signals of the two phases, the elastic body is excited by ultrasonic vibration (traveling wave) in which the antinode and node of the vibration move annularly along the elastic body. The traveling wave causes the rotor, which press-contacts the elastic body, and the driving shaft to rotate.
As described above, the ultrasonic motor produces driving force by frictional force generated by the piezoelectric body and the elastic body, and therefore, when the ultrasonic motor is driven continuously for a long time, there have existed problems in that the piezoelectric body may crack, degradation of performance caused by heat may occur, and elements which form the driving circuit (for example, MOS-FET and the like) may be broken.
In order to solve the above-described problems, generally, there is applied a method in which, with a temperature detecting element such as a thermistor used, when the temperature rises, the driving of an ultrasonic motor is stopped.
However, in the above-described method, there is a possibility that, even if a switch is turned on by an operator, the ultrasonic motor suddenly stops irrespective of whether the ultrasonic motor is broken or not, which makes the operator believe that something is wrong.